Method and device for thermal spraying

ABSTRACT

A thermal spraying method is provided, wherein spray particles of a powdered spray material are introduced into a hot carrier gas stream, heated by the carrier gas stream and then sprayed onto the surface of a substrate by a spray nozzle, wherein the temperature of the spray particles upon impact onto the substrate is below the melting temperature of the spray material. The spray particles are heated in the hot carrier gas stream upstream of the nozzle throat to a temperature that causes at least partial melting of the spray particles in that location.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from German Patent Application Serial No. DE 102012000816.3 filed Jan. 17, 2012.

BACKGROUND OF THE INVENTION

The present invention relates to a method and to a device for thermal spraying wherein spray particles of a powdered spray material are introduced into a hot carrier gas stream, heated by the carrier gas stream and then sprayed onto the surface of a substrate by means of a spray nozzle, wherein the temperature of the spray particles upon impact onto the substrate is below the melting temperature of the spray material, characterized in that the spray particles are heated in the hot carrier gas stream upstream of the nozzle throat to a temperature that causes at least partial melting of the spray particles in that location.

Cold gas spraying is a thermal spraying method in which a powdered spray material (hereinafter termed a “spray particle”) is processed using an expanding gas (hereinafter termed a “carrier gas stream”). The gas is not incinerated thereby. As a rule, 1 to 250 μm spray particles are used and are accelerated in the carrier gas stream to velocities of 200 to 1600 m/s. For this purpose, a Laval nozzle, which has a convergent region and a divergent region, is usually, but not always used. The spray particles are not molten prior to spraying. Upon impact onto the substrate, a coating is formed by plastic deformation and associated heating of the contact zone.

In order to increase efficiency, during cold gas spraying the carrier gas stream can be heated. In the heated carrier gas stream, the particles heat up as well, so that they deform more easily upon impact. The carrier gas temperature, however, is always adjusted so that each and every time, the temperature of the spray particles will be below their melting temperature. The carrier gas stream is thus termed a “cold” gas stream and the method is known as cold gas spraying.

Thus, cold gas spraying is distinguished from other thermal spraying methods by relatively low processing temperatures and high particle velocities. No melting occurs and no phase transformations occur in the coating material; further, the substrate is subjected to only a small thermal load. The coating material barely oxidizes, meaning that layers can be produced, with high spraying efficiency and low spraying losses, which are practically free from pores.

SUMMARY OF THE INVENTION

The present invention proposes a method and a device for thermal spraying wherein spray particles of a powdered spray material are introduced into a hot carrier gas stream, heated by the carrier gas stream and then sprayed onto the surface of a substrate by means of a spray nozzle, wherein the temperature of the spray particles upon impact onto the substrate is below the melting temperature of the spray material, characterized in that the spray particles are heated in the hot carrier gas stream upstream of the nozzle throat to a temperature that causes at least partial melting of the spray particles in that location. Preferred embodiments are described in the description below.

The proposed method concerns a thermal spraying method wherein the spray material is already in the powder form. The method thus differs from methods in which the material to be added is molten, such as flame spraying, plasma spraying and arc spraying. In the method of the invention, energy is supplied by a hot gas, i.e. not by other energy carriers such as a burner flame, an arc, a plasma, a laser beam or the like. The method may be carried out with suitable cold gas spraying units. Thus, the method of the invention closely resembles cold gas spraying as regards carrying it out, but differs from cold gas spraying in distinctive and essential features, as will be explained in more detail below.

In the method of the invention, as is usually the case with cold gas spraying as mentioned above, spray particles of a spray material in powder form are introduced into a hot carrier gas stream, heated in the hot carrier gas stream and then sprayed onto a surface of a substrate by means of a spray nozzle. In traditional cold gas spraying, no melting or fusion of the spray particles occurs. In the method of the invention, however, the spray particles are partially molten and fused. To this end, upstream of the nozzle throat, the particles are heated to a temperature at which the spray particles become at least partially molten. Downstream of the nozzle throat, i.e. in the divergent section of the nozzle, in which the pressure in the carrier gas stream is released, the gas and spray particles cool down. In the method of the invention, this has the result that the spray particles become solid again, since the temperature is below the melting temperature once again. Despite this, when the spray particles hit the substrate they are at a high temperature, since the molten particles—before they cool down—solidify and thus re-release the heat of fusion that was taken up upstream of the nozzle throat upon fusion.

The descriptor “partial melting” may on the one hand include the fact that only some spray particles melt. This may, for example, be the case when spray particles are used that are formed from different materials that have different melting temperatures. The spray particles with a lower melting temperature will then be at least partially liquefied at the appropriate temperatures, whereas the spray particles formed from a higher melting point material will remain in the solid phase. Such “partial” melting may, however, also occur when spray particles with different sizes are used. In this case, any smaller particles may be completely melted, i.e. right to their core, whereas for larger particles, only the periphery will fuse, but the core will remain solid. This is also obviously the case for particles formed from different materials. The descriptor “partial melting” can thus also be understood from this to mean that at least a portion of the spray particles become liquefied at some location. An “at least” partial melting thus also encompasses complete or at least substantial liquefaction of all or at least almost all of the spray particles. As a rule, though, not all of the heat of fusion is provided to the particles, so that complete liquefaction does not occur.

Since the melting temperature and the heat of melting are dependent on the material or the material composition, the temperature to which the spray particles should be heated in order to obtain partial melting will depend on the spray powder itself. The term “hot carrier gas stream”, into which the spray particles are introduced, should therefore be understood to mean a carrier gas stream which has been heated to at least a temperature that corresponds to the melting temperature of the material. In cases in which the spray particles are composed of different materials, this minimum temperature corresponds to the melting temperature of those components which have the lowest melting temperatures. Since the heat has to be transferred from the carrier gas to the spray particles, the required carrier gas temperature is above the minimum temperature. How much the minimum temperature should be exceeded by depends on the heat transfer between the carrier gas and the spray particles and on the residence time for the spray particles in the hot carrier gas. Thus, in some cases it may be sufficient to exceed the minimum temperature by only a few Kelvin, while in other cases the temperature must exceed it by several hundred (or more) Kelvin. This means that the carrier gas temperature can be between 40° C. and 2000° C. The upper limit that is given arises from the limitations of the cold gas spraying unit used for the method of the invention and not from the method per se. The required carrier gas temperature can be determined by calculation and can be ascertained by means of simple series of tests.

The carrier gas temperatures to be used are thus based on the respective spray material and the time for which the particle can be exposed with the appropriate spraying device. This can be determined by calculation and can also be ascertained using routine series of tests. The melting temperatures of the various spray materials are usually known and given by the manufacturer or can be obtained from the appropriate reference books. The exposure time corresponds to the residence time of the particle at the respective temperature. This is in particular a function of the path followed by the particles in the heating zone and also a function of the velocity at which the particles are transported through the heating zone, as well as the type of gas of the carrier gas stream, since heat transfer depends on the gas used.

The fact that the spray particles have partially melted during the method has an effect on the coating itself. As a consequence, it can be concluded from the structure and the properties of the coating whether this has been produced by the method of the invention. If the particles have been partially melted, as is the case in the method of the invention, then upon solidification of the particles, a new structure forms in the molten region such that the structure of the molten and the non-molten regions are different. In traditional cold gas spraying, these differences in the structure are not observed, since the particles do not melt and thus there are no differing regions. In conventional thermal spraying methods, on the other hand, the spray materials are completely molten, and so here again no regions with different textures and properties are formed. The structure and properties can be observed on a section under the microscope, and so the way in which the coating has been produced can be determined from the coating itself.

In traditional spraying methods, in which the spray material hits the substrate in molten form, it is not possible to achieve sufficient oxidation protection without taking complicated additional measures. Metals have a strong tendency to oxidation, in particular in the molten state. This disadvantage arises to a much lesser extent in cold gas spraying, since the particles hit the substrate in a “cold” form, i.e. not in the molten state. Protection against oxidation is also provided with the method of the invention, because the spray particles are only present in a (part) liquid form in a section of the spray nozzle, and when they leave the nozzle, they have advantageously already re-solidified, so that oxidation is at least largely inhibited. Oxidation of the molten particles can be inhibited by a judicious choice of the carrier gas stream, by selecting suitable inert gases for use such as nitrogen, helium or argon or a mixture thereof. The method of the invention thus means that a large amount of energy can be introduced and thus the deformability of the spray particles can be increased without causing excessive oxidation.

Advantageously, nitrogen, helium or air or a mixture thereof may be used as the carrier gas stream in the method of the invention. Furthermore, argon or another gas or a mixture of these gases may be used. If oxidation is to be avoided, then clearly an oxygen-free mixture of gases should be used.

In a nozzle that is normally used for cold gas spraying, the spray particles in the carrier gas stream initially, for example, pass through a convergent region in which the cross-section of the nozzle channel reduces, thereby accelerating the carrier gas stream. After the nozzle throat, which may be an elongated throat section, the convergent region of the nozzle is followed by a divergent region. In the divergent part of the nozzle, the pressure in the carrier gas stream is released, whereupon it accelerates and cools. Since the carrier gas stream cools down, the spray particles also cool down. Even if a divergent nozzle is not used, the temperature of the carrier gas and the spray particles falls after the narrowest cross-section of the nozzle, until it hits the substrate.

As discussed above, the method of the invention comprises adjusting the temperature of the spray particles such that when they impact the substrate, they are below the melting temperature of the spray material. However, because the particles have already been heated until they are partly molten, this temperature is significantly higher than with traditional cold gas spraying methods.

In the context of the present invention, the temperature of the carrier gas stream and/or the pressure at which it is supplied to the spray nozzle, along with the residence time for the spray particles in the hot carrier gas, can be manipulated so that the spray particles can reach any temperature. Thus, if the carrier gas is heated sufficiently and the spray particles are injected such that they reside for a sufficiently long time in the hot gas stream, the spray particles will be partially molten, thereby constituting the method of the invention. Additional heating, for example downstream of the nozzle, is also feasible, but as a rule it is not necessary. Such a method is thus simple and inexpensive to implement, since extant control units can be used. Similarly, the spray particles can be re-heated, for example using microwaves, as disclosed in EP 1 593 437 B1. This means that the energy input can be increased further.

A cold gas spraying unit is thus suitable for the method of the invention, if it is constructed such that it can permit a carrier gas temperature and a residence time for the particles in the hot carrier gas which heats the spray particles sufficiently to provide the conditions discussed above.

Advantageously, the particles are at least partially heated such that their mean temperature upon impact on the substrate is at least 60%, 70% or 80% of the melting temperature of the spray material in Kelvin. This is accomplished by appropriate adjustment of the temperature to which the spray particles is to be heated upstream of the nozzle throat. At 100% of the melting temperature, the particles become liquid, and so as a rule, this value constitutes the upper limit of a suitable temperature range upon impact. If different spray materials are used, then clearly, the cited range will be reached for some of the particles, but not others. For higher melting particles, then, the temperature upon impact onto the substrate can be 50% of the melting temperature in Kelvin, but for lower melting particles, it could be 90% or more. This state of affairs is accommodated by the wording formulated for the temperature, namely that “at least a portion of the spray particles” is at the appropriate temperature upon impact on the substrate.

The influence of heat in any of the process steps during the production and processing of materials as well as its final application are known to be a function of the temperature to which the materials are exposed and the corresponding exposure time. The temperatures can be given in this respect with respect to the melting temperature of the material and in ° C. or K. If a material with a melting temperature of 1000° C. (1273 K) is heated to 500° C. (773 K), then the temperature is 50% of the melting temperature in ° C. and approximately 61% of the melting temperature in Kelvin.

All currently known cold gas spraying methods comprise heating the spray particles essentially to not more than approximately 60% of their melting temperature in Kelvin. As an example, when spraying titanium, which has a melting temperature of 1680° C. (1953 K), as a rule a stream of gas at 1000° C. (773 K) is used. It can be shown experimentally that spray particles with a 20 μm diameter will impact upon the substrate at approximately 530° C. (803 K), i.e. approximately 41% of their melting temperature in K. The temperature of copper particles with a particle size of 20 μm, when employing a gas temperature of 800° C., is 53% of their melting temperature in K upon impact. Zinc, which has a melting temperature of 420° C., has an impact temperature of 63% of the melting temperature in Kelvin upon impact, again when the particle size is 20 μm and with a gas temperature of 400° C. It should be pointed out that for cold gas spraying these temperatures are already very high values; regular values are much lower.

It has been observed that the method of the invention is of particular advantage in the production of layers and components of so-called heat resistant materials. Heat resistant materials are characterized by the fact that their deformability only increases significantly when they are heated to a temperature that is over a value of 0.5 to 0.6 of the melting temperature, i.e. deformability increases sharply from a temperature of 50% to 60% of the melting temperature. Good deformability aids layer formation. With the method of the invention, then, coatings of heat resistant materials can be produced in a particularly effective manner. This also applies for many different materials. It particularly applies to alloys based on iron, nickel and cobalt. In addition, so-called MCrAlYs are included. MCrAlYs are used a great deal in engine and turbine construction. Ni-based alloys in this category are also known as nickel-based superalloys. An example of a typical MCrAlY alloy used in engine and turbine construction has a melting temperature of approximately 1400° C. (1673 K). This alloy only has sufficient deformability beyond a temperature of 730° C. (1003 K), i.e. 60% of the melting temperature), and so the spray particles can only adhere properly to the substrate when they are at a temperature of 730° C. and higher upon impact. The method of the invention ensures that the highly heat resistant materials have this temperature upon impact with the substrate.

A corresponding method can also be used for spraying spray particles that consist of a spray material comprising aluminum, iron, copper, nickel, zinc and/or tin and/or alloys thereof.

The method of the invention is also advantageous for the production of layers and components formed from composite materials because in this manner, a non-metallic component, for example formed from ceramic or graphite, can be particularly well incorporated into the material structure because of the good plastic deformability of the heated metal. The method of the invention also means that relatively coarse and therefore inexpensive particles, which traditionally cannot deform sufficiently and thus do not form thick layers, can be processed. For the same reason, materials with broader particle size distributions can be used, which again has advantages as regards costs.

In addition, the method of the invention is of advantage in the production of layers and components formed from materials which have a vitreous amorphous structure. For this purpose, spray particles formed from materials which have a vitreous structure, in particular synthetics or metallic glasses, are used. Materials with vitreous or even amorphous structures are only plastically deformable above the so-called glass transition temperature. These include, for example, both metallic glasses, in which the individual atoms are largely arranged in an irregular manner, and synthetics with molecular chains which are irregularly arranged. The term “vitreous” thus means that the components, i.e. atoms or molecules, are not regularly arranged as in a crystal lattice, but are irregular, like the atoms in window glass, for example.

Advantageously, in a method in accordance with the invention, a spray nozzle is used in which the carrier gas stream with the spray particles is compressed in a convergent nozzle section and expanded in a divergent nozzle section. A device that may be used in the method of the invention thus, for example, has a Laval nozzle. A Laval nozzle of this type strongly accelerates the spray particles onto the substrate.

To this end, the spray particles are introduced into the gas stream upstream of the nozzle throat of the Laval nozzle, i.e. in or upstream of the convergent region of the nozzle or its narrowest cross-section. In this connection, however, an arrangement such as that disclosed in EP 1 369 498 B1 is also advantageous. By introducing the spray particles in such a manner, the contact time of the spray particles with the gas stream is relatively long and thus a large quantity of energy can be introduced. At the same time, caking of spray particles onto the inner wall of the nozzle is reduced. However, the method of the invention can also be carried out without using a Laval nozzle, since the spray particles already have sufficiently good deformability due to the intense upstream heating, ensuring adhesion to the substrate even without excessive acceleration. This means that the substrate is protected mechanically.

In a further advantageous embodiment, a spray nozzle is used which has an ante-chamber and/or an elongated convergent section for heating the spray particles, as disclosed in EP 1 791 645 B1, for example. If the spray nozzle used has an upstream ante-chamber or the convergent section, for example a Laval nozzle, is sufficiently elongated, then it is certain that, for example, at least 80% of the spray particles will reach a temperature which corresponds to at least 70% of that of the carrier gas stream.

Advantageously, in order to heat the carrier gas stream via which the spray particles are then heated, at least one external gas heater is used. An example of a gas heater that may be used is disclosed in EP 0 924 315 B1. The gas or gas mixture used is held in a pressurized gas vessel and is temporarily stored in a gas buffer vessel. After removing it from the gas buffer vessel, the gas or gas mixture is heated using electrical resistance heating, inductive heating and/or by a plasma burner. Sufficiently strong heating can also be achieved by using a plurality of heaters, in particular pre-heaters and re-heaters such as those described in DE 10 2005 004 117. EP 1 785 679 A1 discloses another heater, which has filaments that can be heated, which could be used.

A heater which has a resistively heatable graphite felt is particularly advantageous. Graphite felts consist of thin fibers of graphite which touch each other when tangled together. If an electrical voltage is applied to a graphite felt by appropriate contact, then despite the fibers being interrupted, a current flows because it can spread via the contact points between the fibers. A graphite felt thus heats up as a whole when current flows through it and it can thus heat up a gas that flows through the graphite felt. Since the graphite fibers in the graphite felt are very thin, the surface area via which the heat is transferred to the gas is very high in total. This allows gases to be heated at high pressures and to high temperatures. Temperatures that can be reached may be over 1500° C. and up to 2000° C.

A method whereby a spray nozzle is used that has a graphite material in at least one region of its inner wall in a contact region with the spray particles is of particular advantage.

Thus, the term “graphite material” as used in the context of this application means any graphite-containing material, including pure graphite as a bulk material, but also, in particular, appropriate composites or coatings. Modified graphites such as vitreous carbon and the like, for example, are also included.

It has been discovered that a graphite material in the cited field of application exhibits a series of advantageous properties that, in particular in combination, can be used to reach the much higher temperatures that have been mentioned. In addition, a graphite material has the advantage that caking of hot spray particles on the inner wall of the nozzle is prevented and thus the (partially) liquid particles can be sprayed.

In particular, a nozzle can be used for the method of the invention which comprises vitreous carbon as the graphite material. Vitreous carbon, also termed glassy carbon, combines vitreous ceramic properties with those of graphite and thus is particularly advantageous. In addition, metallic, partially or completely ceramic spray nozzles and/or spray nozzles with appropriate inserts, for example ceramic nozzles with graphite inserts or metallic nozzles with ceramic inserts, may be advantageous. The appropriate materials can also be applied in the form of coatings, which makes for particularly inexpensive production compared with bulk materials. A bulk material has, for example in the case of graphite, the advantage that its heat conduction properties can be exploited in a particular manner. A suitable nozzle can thus conduct away heat particularly effectively.

An insert or inlay formed from an appropriate material, for example ceramic, graphite or vitreous carbon, can be replaced very easily, for example in the case of wear. Particularly advantageously, graphite materials in the form of composite materials may also be used. They may in this case be materials based on metals and/or plastics.

The device of the invention that is also proposed, in particular in the form of a spray gun with a nozzle comprising graphite material, profits from the advantages of the defined method in the same manner.

It should be understood that the features that have already been mentioned and those which are yet to be mentioned can be used not only in the combinations given in each respect, but also in other combinations or standing alone without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically shows a spray gun that can be configured to carry out the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is illustrated in diagrammatic form in the drawing and an exemplary embodiment will now be described in detail, making reference to the drawing.

FIG. 1 shows a diagrammatic representation of a spray gun with general reference numeral 1. The spray gun 1 has a spray nozzle 10. The spray gun 1 is aimed at a substrate S and has gas inlets 2, 3 via which a gas stream G, in particular a gas stream that has been heated to the temperatures cited above, can be produced. In order to heat a gas stream G, a gas heating device may be provided upstream of the spray gun 1. Further gas inlets 3 may be used to adjust a gas mixture and/or a gas temperature of the gas stream G.

A spray gun 1 may have an external powder conveyor (not shown) into which a portion of the gas stream G is fed, with which the spray particles P are supplied to the spray gun 1. A particle inlet 4 is provided, by means of which spray particles P can be fed to the spray gun 1. For this purpose, a particle feed device in the form of a powder conveyor (not shown) is provided upstream of the spray gun 1, via which a portion of the gas stream G, possibly in (part) heated form, is guided. The carrier gas stream G and the spray particles P enter a mixing chamber 5 that is disposed inside a multi-part housing 6 of the spray gun 1. The housing 6 is shown in partially cutaway view. The mixing chamber 5 may include further devices for mixing the gas stream G and the spray particles P.

On its spray gun side, a spray nozzle 10 has a nozzle inlet 11 and a nozzle opening 12 on the substrate side. A nozzle channel 13 extends between the nozzle inlet 11 and the nozzle opening 12. If the spray nozzle 10 is formed as a Laval nozzle, the nozzle channel 13 has a nozzle throat 14 at a position that is optimized for the flow. The cross-section of the nozzle channel 13 tapers from the nozzle inlet to the nozzle throat 14. The nozzle channel 13 widens from the nozzle throat 14 to the nozzle opening 12 so that a compressed and heated gas stream can be accelerated under the Laval effect. The gas stream with the thus-heated particles P is then sprayed onto the substrate S as a gas-spray particle mixture GP. The inside of the spray nozzle 10 is advantageously provided with a graphite material, in particular between the nozzle throat 14 and the nozzle opening 12.

LIST OF REFERENCE NUMERALS

-   S Substrate -   G Gas stream -   P Particle -   GP Gas-particle mixture -   1 Cold gas spray gun -   2 Gas inlet -   3 Gas outlet -   4 Particle inlet -   5 Mixing chamber -   6 Housing -   10 Spray nozzle -   11 Nozzle inlet -   12 Nozzle opening -   13 Nozzle channel -   14 Nozzle throat 

What we claim is:
 1. A thermal spraying method, wherein spray particles of a powdered spray material are introduced into a hot carrier gas stream, heated by the carrier gas stream and then sprayed onto the surface of a substrate by means of a spray nozzle, wherein the temperature of the spray particles upon impact onto the substrate is below the melting temperature of the spray material, characterized in that the spray particles are heated in the hot carrier gas stream upstream of the nozzle throat to a temperature that causes at least partial melting of the spray particles in that location.
 2. The method as claimed in claim 1, wherein the temperature to which the spray particles are heated upstream of the nozzle throat is adjusted by controlling a temperature of the carrier gas stream and/or a pressure at which the carrier gas stream is supplied to the spray nozzle.
 3. The method as claimed in claim 1, wherein the temperature to which the spray particles are heated upstream of the nozzle throat is adjusted such that the temperature of at least a portion of the spray particles upon impact onto the substrate is more than 60% of the melting temperature of the appropriate spray material in Kelvin.
 4. The method as claimed in claim 3 wherein said temperature is more than 70% of the melting temperature of the appropriate spray material in Kelvin.
 5. The method as claimed in claim 3 wherein said temperature is more than 80% of the melting temperature of the appropriate spray material in Kelvin.
 6. The method as claimed in claim 1, wherein spray particles formed from metallic materials are used and said metallic materials are selected from the group consisting of heat resistant iron-, nickel- and cobalt-based alloys.
 7. The method as claimed in claim 6 wherein said metallic materials are selected from the group consisting of a MCrAlY alloy, aluminum, iron, copper, nickel, zinc and tin and alloys that contain at least one of these elements.
 8. The method as claimed in claim 1, wherein spray particles formed from composite materials are used.
 9. The method as claimed in claim 1, wherein spray particles formed from materials with a vitreous structure are used.
 10. The method as claimed in claim 9 wherein said vitreous structure materials are selected from the group consisting of synthetic glass and metallic glass.
 11. The method as claimed in claim 1, wherein the carrier gas stream containing the spray particles is initially introduced into a convergent nozzle section and then expanded in a divergent nozzle section of the spray nozzle.
 12. The method as claimed in claim 1, wherein a spray nozzle is used that has a graphite material and/or a ceramic material at least in a region of its inner wall in a zone of contact with the spray particles
 13. The method as claimed in claim 1 wherein a spray nozzle is used that consists of a graphite material and/or a ceramic material.
 14. The method as claimed in claim 1, wherein a spray nozzle is used that has an ante-chamber and/or an elongated convergent section for heating the spray particles.
 15. The method as claimed in claim 1, wherein at least one external gas heat supply is provided to heat the carrier gas stream via which the spray particles are heated.
 16. The method as claimed in claim 1, wherein the carrier gas stream is selected from the group consisting of nitrogen, helium, air and a mixture thereof.
 17. A thermal spraying device, characterized by means for introducing spray particles of a powdered spray material into a hot carrier gas stream that heats them up and for spraying them onto a surface of a substrate by means of a spray nozzle, wherein the temperature of the spray particles upon impact onto the substrate is below the melting temperature of the spray material and wherein the spray particles can be heated to a temperature upstream of the nozzle throat that causes at least partial melting of the spray particles in that location.
 18. The device as claimed in claim 17, wherein the spray nozzle has a graphite material and/or a ceramic material in at least a region of its inner wall and/or consists of a graphite material and/or a ceramic material.
 19. The device as claimed in claim 17, comprising a spray nozzle that has an ante-chamber and/or an elongated convergent section of the spray nozzle in order to heat the spray particles.
 20. The device as claimed in claim 17, comprising an external gas heating supply in order to heat the carrier gas stream. 